As I understand, one of the main reasons of being able to edit answers is to keep them up-to-date as technology changes. However, it seems as if while this is a goal, it's not being done very well (possibly because the sites are still relatively new, but also because no one wants to dig up new answers to questions that people don't have anymore).

So here's my suggestion: Allow certain members* to mark old** answers as up-to-date. For every 100*** they mark up-to-date or edit, they earn a badge. As well, the answer is marked with a tagline at the bottom that reads something along the lines of Is Up-to-date As of January 1, 2010 or whatever the date that it was marked happens to be.

*Personally, I'm not sure that this should be done solely by rep, because someone who has high rep and experience in C might come in and change an answer that seems like it's outdated, but the answer in this language (ex. Java) doesn't need changing for reason X. So maybe base it off of tag badges?